The present invention relates generally to microwave radar level gauges. More particularly, the present invention relates to radar level gauge circuitry and methods of operation used to accurately determine the level of a liquid or other material being monitored.
Radar level gauges are in wide use for making non-contact measurements of the level of products such as process fluids and other materials. These devices utilize antennas to transmit electromagnetic waves toward the material being monitored and to receive electromagnetic waves which are reflected at the surface of the material being monitored. Typically, a first or reference signal having a varying frequency is generated and the transmitted electromagnetic waves are produced as a function of the frequency of the reference signal. A second signal is then obtained from the electromagnetic waves reflected by the surface of the material and received by the antenna. The two signals should have substantially the same frequency, but different phases. A phase shift signal is then generated as a function of the phase differences between the reference signal and the second signal over the range of frequencies. The frequency of the phase shift signal is indicative of the distance travelled by the electromagnetic waves between the antenna and the surface of the material being monitored, and thereby of the level of the material. Radar gauges of the FMCW (frequency modulated continuous wave) type are well known as shown, for example in "Mikrowellen messen Fuellstaende" by Prof. Dr. Johanngeorg Otto in Design & Elekronik-Sensortechnik May 13, 1997, vol. 10/1997.
Conventionally, radar level gauges utilize complex and expensive circuitry to remove, from the phase shift signal, frequency components caused by reflection of the electromagnetic waves from surfaces other than the material being monitored. For example, the circuitry attempts to remove interference signals caused by reflections from the waveguide, from the sides of the tank, and from other surfaces which were not the intended target.
Typically, the interference components cannot be completely removed from the phase shift signal. Further, when monitoring the level of a liquid experiencing turbulent conditions, for example, caused by intentional agitation of a liquid, the phase shift signal will include a wide range of frequency components other than the frequency components corresponding to the level of the liquid which would have been detected in the absence of the turbulence. As a result of this and other phenomena associated with a spectrum analysis of the phase shift signal corresponding to a turbulent liquid or to foam covering the liquid, conventional radar level gauges have experienced difficulty in accurately determining the level of the liquid in the tank.